1. Field of the Invention
The present invention relates to an object detector mounted on a vehicle for detecting an object such as a preceding vehicle with the use of light waves.
2. Description of Related Art
As a kind of object detector mounted on a vehicle, there is a known distance measurement device (a laser radar) for measuring a distance from an obstacle such as a preceding vehicle (a vehicle in front) with the use of a laser and the like, for instance, as disclosed in JP-A-2002-031685 (Patent Document 1). The distance measurement device makes a laser diode intermittently emit the light ahead of the vehicle. The distance measurement device senses light reflected by the obstacle in front of the vehicle with the use of a photo sensor. The distance measurement device measures the distance from the obstacle based on a time difference between light emitting timing and light receiving timing.
More specifically, the distance measurement device has a light emitter, a polygon mirror, and a light receiver. The light emitter emits a laser. The polygon mirror is formed in the shape of a truncated six-sided pyramid. The polygon mirror rotates and functions as a scan mirror. The polygon mirror reflects the laser. The light receiver receives the reflected laser. Thus, the laser emitted by the light emitter is reflected and led ahead of the vehicle by the polygon mirror. At that time, the polygon mirror is rotated so that the laser from the emitter strikes the respective side faces of the polygon mirror. Angles, at which the polygon mirror reflects the laser, are adjusted to scan a predetermined area in front of the vehicle with the laser. A reflector of the preceding vehicle is used as a reflection object in the obstacle. The light receiver receives the laser reflected by the reflector to measure the distance.
In the conventional distance measurement device, the laser projected toward the preceding vehicle scans an area (referred to as a first predetermined area, hereafter) of an angle of approximately 4 degrees in a vertical direction and an angle of approximately 36 degrees in a horizontal direction. In the case where the first predetermined area is applied to a vehicle such as a truck, which is large in vehicle height and has a reflector at a high position, the mounting position of the reflector is higher than the area to which the laser is projected. In such a case, there is a possibility that the laser cannot strike the reflector suitably and distance measurement cannot be performed correctly.